The Laboratory of Cell Biology (LCB) conducts basic research on notch proteins, primarily human and murine notch-1. Notch proteins are transmembrane receptors which participate in the control of cell fate determination in several tissues and organs, including among others the hematopoietic, immune and nervous systems. Human notch-1 was originally identified because a mutation resulting in irreversible receptor activation causes about 10% of the cases of T-cell acute leukemia. Furthermore, notch-1 is overexpressed in a number of solid tumors, and monoclonal antibodies to its intracellular region are being studied as diagnostic agents in cervical, colon and lung carcinomas. In adult mammals, notch-1 is expressed in undifferentiated, stem cell compartments in various organs. The only two postmitotic cell types in which it is expressed are mature T cells and neurons. During FY' 98, The LCB has made a number of significant advances in the study of notch-1, which are of direct relevance to its potential biotechnological applications: 1) we completed the development of the expression system originally developed by LM , which was used to produce a biologically active notch-1-derived protein in soluble form with 6 disulfide bonds. This vector has now been made stable in the complete absence of antibiotic selection. This work was submitted for publication and was presented at the FDA forum. (LM, CF); 2) we have shown that notch-1 expression is necessary for the induction of differentiation by experimental anticancer drugs of the hybrid polar family in Friend erythroleukemia cells. This effect of notch-1 is exerted early during exposure to the differentiating agent and is independent of the G1 prolongation which is known to be induced by these drugs. Using antisense techniques, we have shown that notch-1 affects survival of Friend erythroleukemia cells and is potentially an important component in the switch between differentiation, growth and apoptosis. This work has been submitted to Current Biology for publication, was presented at the FDA forum and has led to the filing of a patent application for notch-1 antisense therapy. (LS); 3) the characterizing of novel, biologically active notch-1 monoclonal antibodies with potential clinical applications. A patent application has been submitted (CF) and 4) we are continuing the systematic characterization of the in vivo transgenic model of notch-1 deficiency we generated over the past 3 years (LS, CF, LM, EL). The LCB is now building on these results and using the novel biologics it developed to study: 1) the role of notch-1 in the mechanism of action of other differentiation-inducing chemotherapeutic agents (CF, LS); 2) the function of notch-1 in differentiated neuronal cells in vitro (CF); 3) the level of expression of notch-1 in malignancies other than the ones already characterized, to clarify the possible role of this protein as a tumor marker (CF) and 4) the physiological role of notch-1 in hematopoietic and T cells in the transgenic mouse model we developed (LS, CF, EL, LM).